This invention relates to a brake proportioning system for the rear brakes of an automotive vehicle.
During braking of an automotive vehicle, the brake torque (the torque established by the wheel brakes resisting wheel rotation) generated by a hydraulic brake system causes the wheel to decelerate. As the wheel speed is reduced to a value lower than the speed of a hypothetical free-rolling wheel, wheel slip is established. As a result of wheel slip, a tire torque (the torque tending to decelerate the vehicle while resisting the brake torque) is established. Due to the difference in the normal forces on the front and rear wheels, the rear wheels will tend to experience a higher slip condition for a given brake torque. In order to provide a more balanced braking of the front and rear brakes of a vehicle and to establish desired vehicle braking characteristics and rear wheel braking performance, it is known to proportion the tire torques of the front and rear wheels to establish a desired relationship.
One proportioning technique establishes a rear wheel tire torque that is proportional to the front wheel tire torque according to the tire loading ratio. In other words, the proportion of the front to rear tire torques is established in accord with the ratio of the normal forces acting on the front and rear axles of the vehicle. Realizations of this technique have been pursued using load sensing mechanisms at the vehicle suspension in order to regulate the front and rear brake pressures in accord with the respective normal forces on the front and rear wheels. Other methods approximating the desired tire torques have been pursued using vehicle-deceleration sensitive proportioning valves for brake pressure regulation. Other systems employ a fixed ratio proportioning valve to establish a predetermined ratio between the front and rear brake torque values.
Yet other systems control the brake pressure applied to the wheels so as to establish a controlled slip value between the wheels and the road surface. However, this form of pressure control requires knowledge of vehicle speed during braking. Since the vehicle wheels are all experiencing a degree of slip relative to the road surface during braking, vehicle speed must either be estimated or measured by a transducer such as an accelerometer or a radar system.